The present invention relates to a fan for a turbofan gas turbine engine.
A conventional turbofan gas turbine engine includes a fan, which comprises a fan rotor carrying a single set of circumferentially spaced radially extending fan blades. The ratio of the radius of the radially inner ends of the fan blades to the radius of the radially outer ends of the fan blades, also known as the hub to tip ratio, is about 0.3 for a conventional turbofan gas turbine engine. The problem with this arrangement is that the flow area through the fan is about 90% of the available projected area and thus the mass flow capacity of the fan is limited.
Accordingly the present invention seeks to provide a novel fan for a gas turbine engine which reduces, preferably overcomes, the above mentioned problem.
Accordingly the present invention provides a fan for a turbofan gas turbine engine comprising a fan rotor carrying a first set of circumferentially spaced radially extending fan blades and a second set of circumferentially spaced radially extending fan blades, the second set of fan blades being arranged downstream of the first set of fan blades, the ratio of the radius of the radially inner ends of the first set of fan blades to the radius of the radially outer ends of the first set of fan blades is similar to the ratio of the radius of the radially inner ends of the second set of fan blades to the radius of the radially outer ends of the second set of fan blades, the radius of the radially inner ends of the first set of fan blades is less than the radius of the radially inner ends of the second set of fan blades and the radius of the radially outer ends of the first set of fan blades is less than the radius of the radially outer ends of the second set of fan blades.
Preferably the ratio of the radius of the radially inner ends of the first set of fan blades to the radius of the radially outer ends of the first set of fan blades is between 0.25 and 0.5, preferably 0.3.
Preferably the ratio of the radius of the radially inner ends of the second set of fan blades to the radius of the radially outer ends of the second set of fan blades is between 0.25 and 0.5, preferably 0.3.
Preferably the ratio of the radius of the radially inner ends of the first set of fan blades to the radius of the radially outer ends of the first set of fan blades is substantially the same as the ratio of the radius of the radially inner ends of the second set of fan blades to the radius of the radially outer ends of the second set of fan blades.
Preferably the radius of the radially outer ends of the first set of fan blades is between 40% and 70% of the radius of the radially outer ends of the second set of fan blades.
Preferably the radius of the radially outer ends of the first set of fan blades is 60% of the radius of the radially outer ends of the second set of fan blades.
Preferably the number of fan blades in the first set of fan blades is equal to the number of fan blades in the second set of fan blades.
Alternatively the number of fan blades in the first set of fan blades is equal to a multiple of the number of fan blades in the second set of fan blades.
The number of fan blades in the first set of fan blades may be equal to four times, three times, twice, a quarter, a third or a half of the number of fan blades in the second set of fan blades.
Preferably each of the fan blades in the first set of fan blades is arranged circumferentially between two of the fan blades in the second set of fan blades.
Alternatively each of the fan blades in the first set of fan blades is arranged substantially in the same radial plane as a corresponding one of the fan blades in the second set of fan blades.
Preferably the ratio of the radius of the radially inner ends of the leading edge of the first set of fan blades to the radius of the radially outer ends of the leading edges of the second set of fan blades is between 0.0625 and 0.25, preferably 0.15.